In certain applications industrial steam turbines are connected to a process network with greatly varying steam consumption. An optimal utilization of the turbine then receives that at certain times steam is to be injected into the turbine from the process network, whereas on other occasions steam is to be extracted from the turbine to the process network.
In order to inject and extract steam at an optimal location in the turbine, multi-point injection/multi-point extraction in accordance with known methods for injection/extraction of steam within the turbine technique are utilized. In the case of injection, the optimal point is as high up in the turbine as possible, that is, it takes place at a point in the turbine with high steam pressure. Optimal extraction is carried out in a corresponding manner as far down in the turbine as possible, that is, at a point with low pressure in the turbine.
According to the conventional technique for externally controlled extraction or injection, separate valves are currently used for controlling the steam flows for injecting steam into the turbine, that is, a number of valves equal to the number of injection points are controlled to open and close to control any steam flows to the different injection points. In a corresponding way, any steam flows from the turbine through extraction points in the turbine are controlled by means of separate valves in connection with extraction of steam from the turbine. The number of extraction valves is then equal to the number of steam extraction points from the turbine.
Consequently, with the current technique for controlling injection and extraction of steam into and from a steam turbine, it is necessary to provide a number of valves equal to the sum of the number of injection and extraction points in the turbine. This entails a large cost for the valves and an unnecessarily complicated valve control as well as, in certain cases, disadvantages from the performance point view of.